purefunction-for-adding-days.js

function addDays(date = new Date(), days = 0) {
  const dateClone = new Date(date); // date is an object provided by reference, we need to clone it
  
  dateClone.setDate(date.getDate() + days);

  return dateClone;
}

const tomorrowDate = (date, days = 1) => addDays(date, days);

it("Test tomorrowDate function", () => {
  cosnt testDateString = "2018-01-01T00:00:00.000Z"
  const testDate = new Date(testString);
  const testTomorrowDate = new Date("2018-01-02T00:00:00.000Z");
  
  expect(yesterdayDate(testDateString).toIsoString()).toBe(testTomorrowDate.toIsoString());
  
  expect(yesterdayDate(testDate).toIsoString()).toBe(testTomorrowDate.toIsoString());
})